Radiation-sensitive emulsions employed in photography are comprised of a dispersing medium, typically gelatin, containing radiation-sensitive microcrystals--known as grains--of silver halide. The radiation-sensitive silver halide grains employed in photographic emulsions are typically comprised of silver chloride, silver bromide, or silver in combination with both chloride and bromide ions, each often incorporating minor amounts of iodide.
Radiation-sensitive silver iodide emulsions, though infrequently employed in photography, are known in the art. Silver halide emulsions which employ grains containing silver iodide as a separate and distinct phase are illustrated by Steigmann German Pat. No. 505,012, issued Aug. 12, 1930; Steigmann, Photographische Industrie, "Green-and Brown-Developing Emulsions", Vol. 34, pp. 764, 766, and 872, published Jul. 8 and Aug. 5, 1938; Maskasky U.S. Pat. Nos. 4,094,684 and 4,142,900; and Koitabashi et al U.K. Patent Application No. 2,063,499A. Maskasky Research Disclosure, Vol. 181, May 1979, Item 18153, reports silver iodide phosphate photographic emulsions in which silver is coprecipitated with iodide and phosphate. A separate silver iodide phase is not reported.
The crystal structure of silver iodide has been studied by crystallographers, particularly by those interested in photography. As illustrated by Byerley and Hirsch, "Dispersions of Metastable High Temperature Cubic Silver Iodide", Journal of Photographic Science, Vol. 18, 1970, pp. 53-59, it is generally recognized that silver iodide is capable of existing in three different crystal forms. The most commonly encountered form of silver iodide crystals is the hexagonal wurtzite type, designated .beta. phase silver iodide. Silver iodide is also stable at room temperature in its face centered cubic crystalline form, designated .gamma. phase silver iodide. A third form of crystalline silver iodide, stable only at temperatures above about 147.degree. C., is the body centered cubic form, designated .alpha. phase silver iodide. The .beta. phase is the most stable form of silver iodide.
James, The Theory of the Photographic Process, 4th Ed., Macmillan, 1977, pp. 1 and 2, contains the following summary of the knowledge of the art:
According to the conclusions of Kokmeijer and Van Hengel, which have been widely accepted, more nearly cubic AgI is precipitated when silver ions are in excess and more nearly hexagonal AgI when iodide ions are in excess. More recent measurements indicate that the presence or absence of gelatin and the rate of addition of the reactants have pronounced effects on the amounts of cubic and hexagonal AgI Entirely hexagonal material was produced only when gelatin was present and the solutions were added slowly without an excess of either Ag.sup.+ or I.sup.-. No condition was found where only cubic material was observed.
Tabular silver iodide crystals have been observed. Preparations with an excess of iodide ions, producing hexagonal crystal structures of predominantly .beta. phase silver iodide are reported by Ozaki and Hachisu, "Photophoresis and Photo-agglomeration of Plate-like Silver Iodide Particles", Science of Light, Vol. 19, No. 2, 1970, pp. 59-71, and Zharkov, Dobroserdova, and Panfilova, "Crystallization of Silver Halides in Photographic Emulsions IV. Study by Electron Microscopy of Silver Iodide Emulsions", Zh. Nauch. Prikl. Fot. Kine, March-April, 1957, 2, pp. 102-105.
Daubendiek, "AgI Precipitations: Effects of pAg on Crystal Growth(PB)", III-23, Papers from the 1978 International Congress of Photographic Science, Rochester, N.Y., pp. 140-143, 1978, reports the formation of tabular silver iodide grains during double-jet precipitations at a pAg of 1.5. Because of the excess of silver ions during precipitation, it is believed that these tabular grains were of face centered cubic crystal structure. However, the average aspect ratio of the grains was low, being estimated at substantially less than 5:1.
Prior to the present invention a variety of photographic advantages have been recognized to be attributable to silver halide emulsions containing tabular grains of high average aspect ratios. Kofron et al U.S. Ser. No. 429,407 teaches speed-granularity relationship improvements, increased separation of spectrally sensitized and native speeds, and sharpness advantages for high aspect ratio tabular grain emulsions. Kofron et al further teaches increasing the permissible maximum thickness of the tabular grains to 0.5 micron to increase blue light absorption, recognizing that the thinness of tabular grains reduces their light absorbing capacity in the absence of spectral sensitizing dyes. Wilgus and Haefner U.S. Ser. No. 429,420, Daubendiek and Strong U.S Pat. No. 4,414,310, and Solberg, Piggin, and Wilgus U.S. Ser. No. 431,913 disclose the preparation of high aspect ratio tabular grain silver bromoiodide emulsions, the iodide content being limited by its solubility in silver bromide. Thus, no separate silver iodide phase is present. Abbott and Jones U.S. Pat. No. 4,425,425 discloses reductions in crossover and Dickerson U.S Pat. No. 4,414,304 discloses increased covering power at higher levels of hardening in radiographic elements containing high aspect ratio, tabular grain silver halide emulsions. Wey U.S. Pat. No. 4,399,215 and Maskasky U.S. Pat. No. 4,400,463 disclose high aspect ratio tabular, grain silver chloride emulsions. Mignot U.S. Pat. No. 4,386,156, filed Nov. 12, 1981, discloses high aspect ratio tabular silver bromide emulsions wherein the tabular grains have square or rectangular major crystal faces. High aspect ratio tabular grain silver bromide emulsions wherein the grains have hexagonal major crystal faces are disclosed by de Cugnac and Chateau, "Evolution of the Morphology of Silver Bromide Crystals During Physical Ripening", Science et Industries Photographiques, Vol. 33, No. 2 (1962), pp. 121-125. Jones and Hill U.S. Ser. No. 430,092 now abandoned in favor of U.S. Ser. No. 553,911, filed Nov. 21, 1983, discloses increased speeds with reduced silver coverages in image transfer film units containing high aspect ratio tabular grain emulsions. Evans et al U.S. Ser. No. 431,912 discloses internal latent image forming high aspect ratio tabular grain silver halide emulsions, showing particular advantages in stability and in protection against rereversal. Wey and Wilgus U.S. Pat. No. 4,414,306 discloses high aspect ratio tabular grain silver chlorobromide emulsions. Maskasky U.S. Ser. No. 431,855 discloses epitaxial deposition onto high aspect ratio tabular silver halide grains, with resulting advantages in sensitivity. All of the copending patent applications patents cited above in this paragraph are commonly assigned and, except for Mignot, the filing date of which is given above, were filed on Sept. 30, 1982. None of these copending, commonly assigned patent applications teach or suggest the use of high aspect ratio tabular grain silver iodide emulsions.
House U.S. Ser. No. 451,366, filed concurrently herewith and commonly assigned, titled MULTICOLOR PHOTOGRAPHIC ELEMENTS CONTAINING SILVER IODIDE EMULSIONS, now abandoned in favor of continuing application Ser. No. 543,656, filed Oct. 19, 1983 discloses investigations of high aspect ratio tabular grain silver iodide emulsions in forming emulsion layers of multicolor photographic elements.